An Analysis of a Permissive Overreaching
Transfer Trip Scheme at a 120kV Substation
Russell Louie and Mehdi Etezadi-Amoli
Abstract—This paper describes a post fault investigation into
an undesired operation of a protective device in a permissive
overreaching transfer tripping scheme at a 120kV substation.
The paper includes a system description, an event analysis, and a
proposed solution.
Index Terms—Permissive Overreaching Transfer
(POTT) scheme, Protective relaying, Transmission lines.
S
Trip
I. INTRODUCTION
IERRA Pacific Power Company (SPPC) is the main
power supplier for Nevada and northeastern California.
The Company was founded over 150 years ago and was
responsible for delivering power to a rapidly increasing
number of silver mines in Virginia City. In 1999, SPPC
merged with Nevada Power Company and expanded its
coverage area to over a million customers. Until the 1960s,
nearly all of the power distributed in northern Nevada was
purchased from other suppliers. Presently, SPPC continues to
build generation in an attempt to decrease outside
dependency.
The Reno-Tahoe International Airport is one of the largest
power consumers in the Reno area. It is fed by a 120 kV sub
transmission system, as shown in Fig. 1. In this figure,
significant transmission ties into the rest of SPPCs system at
Steamboat, Mira Loma, and N. Valley are shown as
equivalent generators. During a major storm, the 127 line
tripped at both ends and successfully reclosed automatically.
At the same time, the N. Valley 174 line terminal tripped and
locked out. With the 174 line out of service, loads at Rusty
Spike and Airport are supplied solely from Steamboat and
Mira Loma. Due to the significant load at Airport, this creates
a strain on the system that would require load tripping during
summer peaks.
The main objective of this analysis is to understand why the
N. Valley relay tripped and to determine the proper settings of
the protective scheme to prevent similar undesired operations
in the future.
R. Louie is a graduate student in electrical engineering at the University of
Nevada,
Reno
(UNR),
Reno,
NV
89557-0153
(e-mail:
louier@unr.nevada.edu)
M. Etezadi-Amoli is a professor of electrical engineering at UNR (e-mail:
etezadi@unr.edu)
II. SYSTEM DESCRIPTION
The lines that make up the system in Fig. 1 are protected for
line to ground faults by distance relays and ground
overcurrent relays.
The 127 line is protected by
electomechanical phase distance and ground overcurrent
relays in a POTT scheme with automatic reclosing at both
ends. The 174 line is protected by Schweitzer Engineering
Laboratories (SEL) relays using phase distance, ground
distance, and ground overcurrent elements in a POTT scheme.
The N. Valley terminal uses SEL 321 and SEL 221F relays
while the Rusty Spike terminal uses SEL 321 and SEL 311C
relays. The 172 and 173 lines are both protected by SEL 321
and SEL 311C relays using phase distance, ground distance,
and ground overcurrent elements in a stepped distance
scheme. Automatic reclosing is not used because the majority
of the 173 line and all of the 172 line are underground.
A. Tools for Fault Analysis
SEL relays provide two types of reports for analyzing a
system whenever there is a fault: Tabulated Sequential Event
Repots (SER) and Graphical Event Reports.
The SER provides a display of elements that are asserted
and deasserted during a fault. It is taken from the relay and
relates the time and date to the elements that asserted and
deasserted as the fault took place. Fig. 2 shows the SER
report for the Rusty Spike substation during the storm.
SEL Event Reports were studied to analyze the type of
fault, the fault time, the fault currents, and the behavior of the
relay elements. Figs. 3 and 4 show the Event Reports for N.
Valley and Rusty Spike. A dark line associated with the
element name implies asserted status and a light line implies
deasserted status. The SEL definition keys are as follows:
• N. Valley,
TRIP and 3PT (3 phase trip) are the trips issued by the
relay. KEY and Xmt Perm S (transmit permissive
signal) are different relay elements, but essentially the
same data, transmitting the permissive signal to the
other end of the line. Rcv Perm Sig is the permissive
signal received. 52A is the status of the circuit breaker
(asserted element is a closed breaker).
• Rusty Spike,
52A, TRIP, and KEY are the same as described above
for the N. Valley relay. TMB1A is essentially the KEY
relay element. PT, PTRX, and RMB1A are elements
for the permissive signal received from the other end of
Steamboat
120.kV
Airport
120.kV
Mira Loma
120.kV
173 line
127 line
the line.
Rusty Spike
120.kV
172 line
N. Valley
120.kV
174 line
Fig. 1. System one-line diagram.
RSK 174-21P Date: 12/17 Time: 07:37:06.129
RUSTY SPIKE 174 LINE
FIDSEL-311=C-R105-V0-Z00303-D20011204
CID=98
DATE
12/14
12/14
12/14
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TIME
ELEMENT
12:19:52.865
50P1
12:19:52.865
PTRX
12:19:52.899
KEY
12:19:52.932
50P1
12:19:52.949
PTRX
12:19:52.957
KEY
12:19:54.320
LOP
12:19:54.362
LOP
15:29:02.438
PTRX
15:29:02.472
KEY
15:29:02.488
PTRX
15:29:02.530
KEY
15:37:35.025
KEY
15:37:35.071
KEY
15:37:35.079
PTRX
15:37:35.142
PTRX
15:37:36.234
KEY
15:37:36.288
OUT102
15:37:36.288
OUT103
15:37:36.288
PTRX
15:37:36.304
50P1
15:37:36.334
50P1
15:37:36.334
IN101
15:37:36.338
KEY
15:37:36.350
PTRX
15:37:36.438
OUT102
15:37:36.438
OUT103
15:49:51.213
IN101
16:06:28.318
50P1
16:06:28.326
PTRX
16:06:28.359
KEY
16:06:28.368
50P1
16:06:28.384
PTRX
16:06:28.418
KEY
16:25:23.967
50P1
16:25:23.975
PTRX
16:25:24.008
KEY
16:25:24.017
50P1
16:25:24.038
PTRX
16:25:24.067
KEY
17:03:40.388
KEY
17:03:40.442
OUT102
17:03:40.442
OUT103
17:03:40.442
PTRX
17:03:40.467
KEY
17:03:40.488
IN101
17:03:40.500
PTRX
STATE
Asserted
Asserted
Asserted
Deasserted
Deasserted
Deasserted
Asserted
Deasserted
Asserted
Asserted
Deasserted
Deasserted
Asserted
Deasserted
Asserted
Deasserted
Asserted
Asserted
Asserted
Asserted
Asserted
Deasserted
Deasserted
Deasserted
Deasserted
Deasserted
Deasserted
Asserted
Asserted
Asserted
Asserted
Deasserted
Deasserted
Deasserted
Asserted
Asserted
Asserted
Deasserted
Deasserted
Deasserted
Asserted
Asserted
Asserted
Asserted
Deasserted
Deasserted
Deasserted
12/14
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17:03:40.592
17:03:40.592
17:04:45.845
21:22:23.438
21:22:23.467
21:22:24.617
21:22:24.626
21:22:24.642
21:22:24.667
21:22:24.667
21:22:24.667
21:22:24.705
21:22:24.709
21:22:24.730
21:22:24.817
21:22:24.817
21:24:38.626
06:27:49.779
06:27:49.824
06:27:49.833
06:27:49.895
06:29:24.840
06:29:24.886
06:29:24.894
06:29:24.957
06:29:57.696
06:29:57.725
06:29:58.893
06:29:58.943
06:29:58.943
06:29:58.943
06:29:58.951
06:29:58.984
06:29:59.009
06:29:59.093
06:29:59.093
06:38:05.737
00:00:25.347
00:00:25.406
00:00:25.406
00:00:25.406
00:00:25.452
00:00:25.452
00:00:25.464
00:00:25.556
00:00:25.556
00:17:13.916
Fig. 2. Sequence of Events Report.
OUT102
OUT103
IN101
KEY
KEY
KEY
50P1
50P1
OUT102
OUT103
PTRX
KEY
IN101
PTRX
OUT102
OUT103
IN101
KEY
KEY
PTRX
PTRX
KEY
KEY
PTRX
PTRX
KEY
KEY
KEY
OUT102
OUT103
PTRX
KEY
IN101
PTRX
OUT102
OUT103
IN101
KEY
OUT102
OUT103
PTRX
IN101
KEY
PTRX
OUT102
OUT103
IN101
Deasserted
Deasserted
Asserted
Asserted
Deasserted
Asserted
Asserted
Deasserted
Asserted
Asserted
Asserted
Deasserted
Deasserted
Deasserted
Deasserted
Deasserted
Asserted
Asserted
Deasserted
Asserted
Deasserted
Asserted
Deasserted
Asserted
Deasserted
Asserted
Deasserted
Asserted
Asserted
Asserted
Asserted
Deasserted
Deasserted
Deasserted
Deasserted
Deasserted
Asserted
Asserted
Asserted
Asserted
Asserted
Deasserted
Deasserted
Deasserted
Deasserted
Deasserted
Asserted
Fig. 3. SEL report of N. Valley.
Fig. 4. SEL report of Rusty Spike.
A. Event Description
Relay targets indicated that a temporary line to ground fault
occurred on the 127 line. The relays at Mira Loma and
Steamboat successfully tripped and reclosed after the fault
cleared. At the same time, the N. Valley 174 terminal tripped
while the Rusty Spike terminal remained closed. Because of
its trip time, it was initially suspected that the N. Valley relays
misoperated due to an instantaneous overreaching element.
However, as shown in this paper, this was not the reason for
the undesired operation.
Understanding the behavior of the relays in this system is a
vital part in solving this problem. The electromechanical KD
and CEY relays on the 127 line detect ground faults in a zone
scheme. In a 3-zone scheme, the first zone covers up to 80%
of the line, the second zone covers 120% of the line, and the
third zone covers beyond the second zone and into the third
line. The timing of these zones is such that the shorter zones
trip at shorter times. Also, a communication procedure was
implemented into the adjacent relays to coordinate the proper
trip sequence. This communication allows the breakers
closest to the fault to trip first, minimizing the outage as much
as possible. SEL relays work much like the eletromechanical
relays with the exception that they are able to help locate
faults and read fault currents and provide this information
along a 60 Hz time axis. This makes it possible to accurately
analyze a particular relay operation. Clearly, the overcurrent
relays operate according to fault current characteristics of the
line, and are coordinated with the other overcurrent protective
elements.
III. EVENT ANALYSIS
In order to determine why the N. Valley 174 terminal
tripped, the following three possibilities were analyzed.
a.
b.
c.
The fault occurred in the instantaneous protection zone
of the N. Valley relay, causing it to trip without delay.
An underreaching reverse distance element at Rusty
Spike prevented the blocking capability of an echo
keying scheme.
An inactive or improperly coordinated reverse reaching
ground overcurrent element at Rusty Spike prevented
the blocking capability of an echo keying scheme.
IV. SOLUTION
The first possibility assumes that the phase to ground fault
occurred at the closest distance to the Mira Loma substation.
With this assumption, N. Valley would see the minimum
possible impedance with the maximum amount of fault
current. The impedance between Mira Loma and N. Valley
was found as follows:
Z (Primary)NVR = Z 174Line(pu) + Z 172Line(pu) + Z 173Line(pu)
(1)
= (0.0098 + j 0.0392)
(2)
(3)
Therefore:
Z (Primary)N VR = 0 .0404 p.u.
(4)
= 0.0017 + j 0.0081 + 0.003 + j 0.0073 + 0.0046 + j 0.0239
Using the PT (1000:1) and CT (1200:5) ratios and converting
all values to ohm we have:
Z (Secondary)NVR = 0.0404 ×
(1200 / 5) 120 2
×
= 1.3970Ω
1000
100
(5)
The setting of the instantaneous ground distance element
(zone 1) at N. Valley was 0.18 Ω. Because the value
calculated in equation 5 is well beyond the reach of N.
Valley’s zone 1 setting, there is no way this element could
have caused the undesired trip. After simulating a fault on the
Mira Loma bus, it was determined that the N. Valley ground
relay would see about 1600 A primary or about 6.7 A
secondary. The instantaneous ground overcurrent element at
N. Valley was set at 50 A secondary (12000 A primary). With
this setting, a fault anywhere on the 127 line would not have
been picked up by this element. Because neither of the N.
Valley instantaneous elements could have seen the fault on
December 14th, it was concluded that the first possibility could
not have happened.
The second possibility was that the misoperation was
caused by an underreaching reverse distance element at Rusty
Spike designed to block the echoing capability of the relay.
The N. Valley 174 line has an active POTT scheme in
addition to the usual direct tripping elements. As a result, it
will trip when its communications scheme trip mask (MTCS)
setting is asserted and it also receives a permissive trip from
Rusty Spike.
MTCS at N. Valley has the following elements:
MTCS=M2P+Z2G+51NP
Therefore, a fault in the zone 2 phase distance (M2P), zone
2 ground distance (Z2G), or ground overcurrent (51NP)
elements will cause the relay to trip if a permissive signal is
received.
POTT schemes require permission from both terminals of a
line to achieve faster trip times for an internal fault. When
one of the line terminals is open, its protective elements are
unable to detect a fault and cannot send the permission to trip
to the other terminal [3]. In order to overcome this, the SEL
relays have the ability to echo a received permissive signal.
The signal is echoed if the following two conditions are met
[3]:
1) Permissive signal must be received for a set amount of
time.
2) No reverse fault detected by reverse reaching elements.
The trip logic in Fig. 5 shows that in order for a trip to
occur, the PTRX (permissive) element and a forward element
of zone 2 must be asserted, and the Z3RB (reverse reaching
zone 3 elements of other terminal) must be deasserted.
For the N. Valley relay, as long as the MTCS is asserted,
the permissive signal is sent. Therefore, if the fault is seen by
its zone 2 ground distance element (Z2G), the permissive
signal is sent to Rusty Spike. If Rusty Spike’s reverse
reaching zone 3 distance element can not see the fault, it will
echo the permissive signal back to N. Valley, allowing it to
trip. A simulation of the N. Valley and Rusty Spike ground
distance elements is shown in Fig. 6. It can be seen that the
reverse reaching element of Rusty Spike easily overreaches
the zone 2 element of N. Valley. Therefore, the second
possibility could not have happened.
The final possibility was that undesired trip was caused by
an inactive or improperly coordinated zone 3 reverse reaching
ground overcurrent element at Rusty Spike. As described for
the second possibility, this zone 3 element is designed to
block the echoing of the permissive signal. All overreaching
overcurrent and impedance (distance) elements that initiate
permission to trip must be coordinated with the remote-end
reverse reaching elements to ensure that they do not overreach
the reverse elements. It is important to enable and use the
same types of relay elements in the reverse directional relay
for blocking as are used in the forward directional relay for
keying permission [5]. Therefore, if the ground overcurrent
element (51NP) of the N. Valley MTCS sees a fault, the
reverse reaching ground overcurrent element at Rusty Spike
must also see the fault in order to block echoing of the
permissive signal.
The ground overcurrent element at N. Valley is directional
and taken from the 51NP setting. The 51NP element has a
setting of 480 A, which covers most of the 127 line.
Therefore, MTCS will be asserted for nearly any fault on the
127 line. With MTCS asserted, the permissive signal is sent
to Rusty Spike. Rusty Spike requires that the permissive
signal be received for at least two cycles before echoing. As
shown in Fig. 3, the December 14th fault lasted for
approximately four cycles. With the first condition met, the
only thing capable of preventing Rusty Spike from echoing
the signal is its zone 3 reverse reaching ground overcurrent
element.
After reviewing the setting sheets for the Rusty Spike 174
relay, it was determined that the reverse reaching ground
overcurrent element (50G3P) was not activated. Because of
this, there was no echo blocking for anything picked up by the
N. Valley ground overcurrent element (51NP). Since the
50G3P element is used strictly for blocking permissive signal
echoing, a very liberal setting can be used to ensure it
overreaches the 51NP element of N. Valley.
V. CONCLUSION
An investigation and analysis into an undesired trip of a
protection scheme at a 120 kV substation has been presented.
Based on the timing of the relay operation at N. Valley, it was
initially assumed that the relay misoperated by overreaching.
However, after an extensive analysis, it was determined that
this relay operated correctly and that the trip was caused by
the remote terminal’s inability to correctly block echoing of a
permissive trip signal. The use of advanced communication
schemes can lead to overlooking critical elements and
undesired trips. In this particular case, a commonly unused
zone 3 reverse reaching ground overcurrent element was
ignored and an unnecessary outage occurred.
To correct the undesired operation of any given relay
scheme, it is critical to understand the function of the
protection scheme, the relays being used, their settings, and
the output from fault recording devices. This information will
allow correct diagnosis for a given undesired operation and
lead to proper settings. It is essential to carefully choose a
coordinated protection scheme impervious to system
variances.
ACKNOWLEDGMENT
We like to thank Mohammed R. Hashemi, Jason
Gunawardena, and Holly Hoff for their contribution to this
project.
We also like to thank Gene Henneberg, a consulting
engineer at Sierra Pacific Power, for answering our questions
and his valuable knowledge and advice.
Fig. 5. Trip Logic for SEL 321 [3].
NVR1 74-2 1G1
321 -1 Ty pe=SEL 321 MHO4
PTR=1 000:1 CTR=240:1 Min I= 5.0 0A
Zone 1: Z=0. 18 s ec Oh m @ 78.1 deg. T=0.0s
Zone 2: Z=0. 44 s ec Oh m @ 78.1 deg. T=0.60 s
Zone 3: Z=0. 20 s ec Oh m @ 258.1 deg . T=0.1 0s
Zone 4: Z=0. 77 s ec Oh m @ 78.1 deg. T=0.90 s
Line Z= 0. 28@ 7 8.8 sec O hm ( 1. 17 O hm)
RSK1 74-2 1G1
Type=SEL 321 MHO4
PTR=1 000:1 CTR=200:1 Min I= 2.5 0A
Zone 1: Z=0. 14 s ec Oh m @ 64.2 deg. T=0.0s
Zone 2: Z=0. 35 s ec Oh m @ 64.2 deg. T=0.67 s
Zone 3: Z=0. 20 s ec Oh m @ 244.2 deg . T=0.1 7s
Line Z= 0. 23@ 7 8.8 sec O hm ( 1. 17 O hm)
Fig. 6. Distance settings at N. Valley and Rusty Spike [1].
REFERENCES
[1]
[2]
[3]
[4]
[5]
ASPEN Software, Academic Version, 1999, ASPEN, San Mateo, CA
94401.
M. Etezadi-Amoli, R.J. Salgo, “Protective System Performance
Analysis”, Proceedings of the 1998 World Automation Congress, pp.
213.1-213.8, May 1998.
A. Guzman, J. Roberts, K. Zimmerman, “Applying the SEL-311 Relay
to Permissive Overreaching Transfer Trip (POTT) schemes”, SEL
Application Guide, Pullman, WA, 1999.
SEL-311C Instruction Manual, Schweitzer Engineering Labs, Pullman,
WA, 1988.
Application Guide for Echo Keying Logic on Permissive Overreaching
Transfer Trip Schemes, WECC Relay Work Group, May 5, 2008.
Russell Louie received a BSEE in May 2008 from the University of Nevada,
Reno. He currently works for Sierra Pacific Power Company and is
continuing his graduate studies at the University of Nevada, Reno. Russell
was born in Oregon and has lived in Fallon, Nevada for 18 years.
Mehdi Etezadi-Amoli received a BSEE in 1970, MSEE in 1972, and Ph.D.
degree in 1974 from New Mexico State University. From 1975-1979 he
worked as an assistant professor of Electrical Engineering at New Mexico
State and the University of New Mexico. From 1979-1983 he worked as a
Senior Protection Engineer at Arizona Public Service Company in Phoenix,
AZ. In 1983 he joined the faculty of the Electrical Engineering Department at
the University of Nevada, Reno where he is responsible for the power system
program. His present interest is in power system protection, large-scale
systems, fuzzy control, neural network applications, and renewable energy.
Dr. Etezadi is a Registered Professional Engineer in the states of Nevada and
New Mexico.